1. Field of the Invention
The invention relates to the mining of subterranean solid sulphur by the Frasch method in which hot water is sent below to contact the sulphur and heat and liquefy it, the sulphur in liquid form is brought to the surface, and the hot water accumulates above at least a part of the subterranean solid sulphur. The invention more specifically relates to the conservation and utilization of the heat energy contained in the accumulated subterranean hot water.
2. Description of the Prior Art
Sulphur occurs in the caprock of certain salt domes along the coastal area of the Gulf of Mexico and in the offshore waters of the Gulf. The salt domes are believed to have been formed by the intrusion of salt from extremely deep lying beds of salt into the sedimentary formation of the region. The salt intrusives are circular or elliptical in cross-section and the tops of the domes vary in depth below mean sea level from less than one hundred feet to several thousand feet. The tops of some salt domes are capped with limestone, anhydrite, gypsum, or a combination of these minerals. A section of a typical salt dome is shown in FIG. 1.
In the caprock of some of the domes of this type containing limestone with other minerals present, sulphur is found in fissures, cracks, and seams. Sulphur is also occasionally present to a lesser extent in the gypsum and anhydrite associated with the limestone of the dome. All salt domes which are below the surface of the area in which they lie are covered with a layer of shale or other sediments which in essence forms the caprock into a closed container. The sulphur formation is often sandwiched between a layer of overlying barren limestone and an underlying layer of anhydrite. Underneath the anhydrite is the salt proper. Fewer than 10% of the salt domes discovered so far in the coastal region of the Gulf of Mexico have contained sulphur that is economically minable in commercial quantities.
After a dome has been discovered and has been proven to possess economically minable commercial quantities of sulphur in the caprock, the Frasch system of mining is usually initiated. In a typical system, a hole is drilled to a selected zone in the sulphur-bearing limestone by means of oil field type drilling equipment. The well, after drilling, usually is equipped with three concentric pipes within a protective casing which is cemented into the top of the caprock. Inside the outer casing a six-inch pipe is sunk through the caprock to the bottom of the sulphur deposit. The six-inch pipe is perforated with small holes in its lower end portion. Then a three-inch pipe is lowered to a point spaced a short distance from the bottom. Last, and innermost, is a one-inch pipe carrying compressed air and reaching more than half way to the bottom of the well.
Water, heated under pressure to about 325.degree. F. (well above the normal 212.degree. F. boiling point) is pumped down the space between the six-inch and three-inch pipes, and, during the initial heating period described above, also down the three-inch pipe. The initial heating period can extend for periods of 24 to 96 hours and water is injected during this period at the rate of 250 to 750 gallons per minute. The superheated water flows out the holes at the bottom of the six-inch pipe into the sulphur bearing deposit and moves upwardly because of the lower density of the hot water compared to the colder connate water. As the temperature of the sulphur bearing formation reaches and exceeds the melting point of the sulphur, liquid sulphur flows to the bottom of the well, as it is about twice as heavy as water. The pumping of water down the three-inch pipe is then discontinued. Static pressure of the hot water forced into the formation, plus imposed pressure within the dome, then forces liquid sulfur several hundred feet up the three-inch pipe. Compressed air forced down the smaller pipe aerates and lightens the liquid sulphur in the three-inch pipe so that it will rise the rest of the way to the surface. A single well can take the sulphur from only about a half acre of dome area. So new wells must be drilled continually, and new pipelines laid to bring in water and air and carry off the molten sulphur. Other pipe sizes may be used, but this in no way changes the general theory of Frasch mining. A mining system of the Frasch type is disclosed in U.S. Pat. No. 1,612,453.
Since the caprock of the salt dome is essentially a closed container, the injection of hot water for mining purposes will build up the pressure in the dome unless relieved. Relief is accomplished by drilling "bleedwells" to the floor of the dome and removing cold water. Cold water--as cold as practical--is removed to conserve heat in the dome and to maintain the desired mine pressure. In the course of time, large quantities of hot water accumulate in the upper regions of the dome in the barren areas and leached areas (i.e., areas from which substantially all of the sulphur has been removed). The injection of millions of gallons of hot water (325.degree. F.) into the hydraulically closed domes, with the removal of cold water for pressure control, has resulted in the accumulation of trillions of BTU's of heat within the domes. The fluid densities within the caprock are such that the hot water rises as it exits the well bottom, and percolates upwardly through the sulphur-rich limestone. Although the water is cooling as it gives up its heat to the melting sulphur and the surrounding formations, it is still very hot when it enters the formations above the sulphur ore. In fact, temperatures in the range of 220.degree. F. to 290.degree. F. are frequently measured in this spent water near the top of caprock. From this description, it can be seen that, in a typical Frasch process mine, a large limestone caprock exists with the lower portions containing elemental sulphur enrichment within the limestone matrix and with the upper portions containing a vast induced geothermal resource of hot water accumulated from past and ongoing mining operations. A large group of wells can be drilled into the lower portion of the caprock with virgin hot water being injected to continue the sulphur melting process.
The large volumes of water removed through the bleedwells, usually located on the flanks of the dome in order that the water removed is the coldest possible water in the formation, can present a problem. The disposal of this water is a primary environmental problem.
Water, thus returned to the earth's surface (hereinafter designated as bleedwater), is still at an elevated temperature, and in addition to the usual constituents of ground waters such as chlorides, sulphates and bicarbonates of sodium, calcium and magnesium, contains hydrogen sulphide, thiosulphates, hydrosulphides and polysulphides, and other sulphur compounds of various basic elements (which will be designated hereinafter simply as sulphides) and various other dissolved substances. The sulphides and the other constituents in the bleedwater render it highly corrosive and extremely destructive to the usual materials encountered in the commercial operation of a sulphur mine, making its re-use entirely uneconomical.
The re-use, without treatment, of the bleedwater would require all conduits and equipment with which it came in contact to be made of special noncorrosive material, which is extremely costly. Commercial recovery of the heat in bleedwater has been attempted using a closed type of heat exchanger in which the bleedwater flows over conduits carrying colder fresh water, the heat transfer being from the bleedwater through the conduit material and into the fresh water. Only a small part of the heat can be commercially recovered in this manner and the heat exchangers must be constructed of costly non-corrosive materials.
The fundamental problem in the re-use of hot bleedwater for mining is that not only the bleedwater reheating plant, but also the water distribution pipelines and production wells are subjected to severe corrosion and scaling. U.S. Pat. No. 2,109,611 illustrates one attempt to treat bleedwater to render it suitable for re-use. Where, in some cases, the corrosion problem might be reduced by the use of additives, or chemical treatment, these methods result in substantially higher operating expenses.
It has, therefore, been customary to attempt to locate the bleedwells so as to return to the surface as cold a bleedwater as possible and to discharge this water to waste. This practice has accounted for great losses by the sulphur mining industry in the past because of the non-recovery of the heat from the bleedwater, by the inability to re-use the bleedwater and by loss of heat due to the flow of the hot mine water to the upper formations of the deposit where it is no longer available for melting sulphur in situ. The disposal of the bleedwater thus produced is subject to the further disadvantage that the suspended and dissolved matter, hydrogen sulphide and metal sulphides contaminate the surface water into which the bleedwater may be permitted to flow. In order to prevent objectionable pollution, then, it is necessary to purify the bleedwater before its discharge. The apparatus and process of purification before disposal impose a heavy expense upon the sulphur mining industry.
The re-use of bleedwater brought to the surface could provide tremendous economic and environmental benefits. Several unsuccessful efforts have been made in this direction. Excessive scaling and corrosion, however, have caused many of these operations to be uneconomical.
In some instances in the past sulphur wells have been pumped with their liners bleeding water to the atmosphere, creating concurrent flows of sulphur and water. Heat is supplied to such wells by injecting hot minewater down through the caprock casing. The very aggressive nature of the bleedwater renders this technique highly corrosive to the liner and the sulphur delivery pipe. In addition, a considerable quantity of heat is lost to the atmosphere and the large amounts of bleedwater at the surface present serious pollution problems.
U.S. Pat. Nos. 3,525,550; 3,432,205; and 3,258,069 describe methods and apparatus which seek to take advantage of natural hot geopressured aquifers disposed below subterranean sulphur deposits and depend upon special geological formation that may be rare or difficult and expensive to locate. U.S. Pat. No. 3,432,205 circulates hot water down one well and through the sulphur-bearing formation to a second well, upwardly through which moves a mixture of sulphur and hot water and produces large amounts of corrosive water which must be disposed of. U.S. Pat. No. 3,630,573 describes an attempt to reduce the amount of hot water injected into a sulphur well by separately injecting superheated steam and hot water. U.S. Pat. No. 1,339,621 discloses an air lift specially designed in the shape of a Venturi to give rising molten sulphur an extral lift. The methods and apparatus of these patents, however, do not utilize subterranean recycling of hot water to conserve and use the large amount of heat in the subterranean accumulated hot water resulting from subterranean sulphur mining operations.
U.S. Pat. No. 3,938,592 describes a method for extracting heat from subterranean rock strata which have been fractured by one or more explosions and filled with stratal fluid to absorb heat from the rock strata. The heated stratal fluid is then recycled upwardly to heat a heat-carrying agent which is recycled through a heat exchanger to a surface plant where the heat values may be utilized. U.S. Pat. No. 3,333,638 refers to the disposal of water from a gas-producing zone to a lower water-absorbing zone. U.S. Pat. No. 3,515,213 refers to the recovery of shale oil by circulating hot water from the surface through the shale and back to the surface again using two wells. U.S. Pat. Nos. 2,742,091; 2,871,948; 2,980,184 and 3,322,195 all relate to various treatments of oil wells to rejuvenate them and obtain additional production. U.S. Pat. No. 2,742,091 discloses a method in which hot oil is recycled within the well or casing. None of these patents disclose or suggest the mining of sulphur wherein the subterranean hot water accumulations from previous mining operations are subterraneanly recycled downwardly to heat underlying sulphur for liquefying it.